SIADH

Question 1

Define SIADH?

Answer 1

Characterised by continued secretion of ADH, despite the absence of normal stimuli for secretion (i.e. increased serum osmolality or decreased blood volume)

Question 2

What are the risk factors for SIADH?

Answer 2

age >50 years

pulmonary conditions (e.g., pneumonia)

nursing home residence

postoperative state

malignancy- lung malignancy (especially small cell lung cancer), gastrointestinal or genitourinary malignancy, lymphoma, or sarcoma.

Drugs associated with SIADH- SSRIs, amiodarone, carbamazepine, chlorpromazine, amitriptyline, NSAIDs, and chemotherapeutic agents.

CNS disorders- CNS infection, brain masses, cerebral trauma, or cerebrovascular accident.

Question 3

what drugs are associated with SIADH?

Answer 3

SRIs, amiodarone, carbamazepine, chlorpromazine, amitriptyline, NSAIDs, and chemotherapeutic agent

Question 4

what are the causes of SIADH?

Answer 4

Question 5

summarise the epidemiology of SIADH?

Answer 5

Hyponatraemia is the MOST COMMON electrolyte imbalance seen in hospital

< 50% of severe hyponatraemia is caused by SIADH

Question 6

What are the presenting syptoms of SIADH?

Answer 6

• Mild hyponatraemia may be ASYMPTOMATIC
• Headache
• Nausea/vomiting
• Muscle cramp/weakness
• Irritability
• Confusion
• Drowsiness
• Convulsions
• Coma

Question 7

What are the signs of SIADH on physical examination?

Answer 7

MILD hyponatraemia - no signs

SEVERE hyponatraemia:

Reduced reflexes

Extensor plantar reflexes

Question 8

what is the hyponatraemia in SIADH caused by?

Answer 8

dilution from excess water reabsorption-> Not due to decrease in total body Na+

Question 9

why is there euvolaemic hyponatraemia in SIADH?

Answer 9

hypervolaemia causes heart distension so BNP and ANP are secreted which cause salt and water excretion-> ends up EUVOLAEMIC

Question 10

Outline a management plan for SIADH?

Answer 10

1st line: Treat underlying cause

1st line: Fluid restriction to 1L per day

2nd line: Vasopressin receptor antagonists (e.g. tolvaptan)

3rd line: NaCl + furosemide

4th line: demeclocycline

In SEVERE/ACUTE cases - slow IV hypertonic saline and furosemide with close monitoring

Question 11

what are the possible complications of SIADH?

Answer 11

Convulsions

Coma

Death

Central pontine myelinolysis (demylination of pons) - occurs with rapid correction of hyponatraemia Characterised by:

• Quadriparesis
• Respiratory arrest
• Fits

Question 12

summarise the prognosis of SIADH?.

Answer 12

Depends on the CAUSE

Na+ < 110 mmol/L is associated with a HIGH MORBIDITY and MORTALITY

50% mortality with central pontine myelinolysis

Question 13

what are the appropriate investigations for SIADH ?

Answer 13

serum sodium- low

Serum osmolality- low

Serum urea- low

Urine osmolality

urine sodium

Creatinine (check renal function)

Glucose, serum protein and lipids - to rule out pseudohyponatraemia

Free T4 and TSH - hypothyroidism can cause hyponatraemia

Short synacthen test - adrenal insufficiency can cause hyponatraemia

Question 14

what are the results for the investigations for SIADH and therefore its diagnosis

Answer 14

Low plasma osmolality

Low serum Na+ concentration

High urine osmolality

High urine Na+

The presence of the above results and the absence of hypovolaemia, oedema, renal failure, adrenal insufficiency and hypothyroidism are required for the diagnosis of SIADH

Investigations for identifying the cause (e.g. CXR, CT, MRI)

Question 15

what is pseudohyponatraemia?

Answer 15

sodium concentration is actually normal but is erroneously reported as being low because of the presence of either hyperlipidaemia or hyperproteinaemia